<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html lang="en">
<head>
	<meta http-equiv="Content-Type" content="text/html;charset=UTF-8">
	<title>jiedian</title>
</head>
<style type="text/css">
	.son{
		width: 200px;
		height: 200px;
		background: blue url() 0 0 no-repeat;
	}
	.box{
		width: 400px;
		height: 400px;
		background: red url() 0 0 no-repeat;
	}
	
</style>
<script type="text/javascript">
	window.onload = function  () {
		son = document.getElementsByClassName("son")
		son[0].style.backgroundColor="yellow"
	}
</script>
<body>
	<div class="box">
		<div class="son">
			666666666666666666
		</div>
	</div>
</body>
</html>